Liquid crystal display device with a backlight unit having a diffusion plate

ABSTRACT

A liquid crystal display device having a direct backlight which uses a high-bright light source achieves both of a high efficiency and thin uniformity. In a liquid crystal display device which includes a liquid crystal panel, and a backlight unit which is arranged on a side of the liquid crystal panel opposite to a display screen of the liquid crystal panel, the backlight unit includes a housing, a plurality of light sources arranged in the inside of the housing, and a diffusion plate which is arranged between the plurality of light sources and the liquid crystal panel, the diffusion plate includes a plurality of light blocking regions at positions corresponding to the plurality of respective light sources, and the light blocking region at a center portion of the housing and the light blocking region at an edge portion of the housing exhibits transmissivities different from each other.

The present application claims priority from Japanese applicationsJP2006-182951 filed on Jul. 3, 2006, the content of which is herebyincorporated by reference into this application.

BACKGROUND OF THE INVENTION

The present invention relates to a liquid crystal display device, andmore particularly to a technique which is effectively applicable to adirect backlight used in a liquid crystal display device.

A TFT (Thin Film Transistor)-method liquid crystal display module hasbeen popularly used as a display device of a liquid crystal televisionreceiver set, a personal computer or the like.

The liquid crystal display module is constituted of a liquid crystalpanel which arranges a drain driver and a gate driver on a peripherythereof and a backlight which radiates light to the liquid crystalpanel.

The backlight is roughly classified into the side-light backlight and adirect backlight. Recently, along with remarkable spreading of liquidcrystal television receiver sets, large sizing and the acquisition oflarge screen have been in progress with respect to a liquid crystaldisplay module used in a liquid crystal television receiver set or thelike. In such a large-sized and large-screen liquid crystal displaymodule, the direct backlight which can acquire high brightness isadopted.

As a light source of the direct backlight, a cold cathode fluorescentlamp (CCFL) has been dominantly used. Although the CCFL exhibits a longlife time, a tube diameter of the CCFL is small and hence, along withthe progress of large-sizing of a screen, it becomes difficult to adoptthe CCFL as the light source. Accordingly, recently, to sufficientlycope with the large-sized large-screen liquid crystal display module,there exists a demand for the application of a hot cathode fluorescentlamp (HCFL).

The HCFL possesses a large tube diameter compared to the CCFL andexhibits high brightness and hence, the HCFL can realize a backlight fora large screen with the number smaller than the number of the CCFL.However, since the number of the HCFL is small, there arises a drawbackon brightness irregularities.

As means which can efficiently reduce the brightness irregularities whenthe number of fluorescent lamps is small, there has been proposed atechnique which arranges the fluorescent lamps non-uniformly to achievethe brightness distribution in which the center of the screen exhibitshigh brightness and a peripheral portion of the screen exhibits the lowbrightness.

Further, as another means to overcome the brightness irregularities offluorescent lamps, there has been known an example which uses a lightcurtain (see following patent document 1).

[Patent Document 1] JP-A-2005-117023

SUMMARY OF THE INVENTION

In an attempt to realize the distribution which exhibits high brightnessat the center of the screen using the HCFL, due to the relationshipbetween the brightness and the size, the use number of fluorescent lampsbecomes smaller than the use number of the CCFL and hence, it isdifficult to achieve the high brightness at the center of the screen bymerely changing the arrangement position of the fluorescent lamps.Accordingly, the use of the light curtain with the fluorescent lamps isconsidered.

Most of the above-mentioned conventional light curtains have beenstudied on a premise that the CCFL is used as the fluorescent lamp.However, in the CCFL which exhibits low brightness efficiency, the useof the light curtain lowers the brightness and hence, the CCFL tends toobject to the use of the light curtain and avoid the use of the lightcurtain per se. To the contrary, the HCFL can acquire the sufficientlyhigh brightness and hence, the HCFL can acquire the sufficientbrightness even when the light curtain is used.

However, the light curtain described in the above-mentioned example ofthe related art is a technique which makes the brightness uniform bycontrolling the distribution of transmissivity using a dot patternhaving dots of different diameters. Such a dot pattern is, when the highbrightness light source having a tube diameter which exceeds 10φ, forexample, the HCFL is used, insufficient to make the brightness uniform.This is because that the dot pattern is usually formed by printing areflective material and hence, density in printing is limited. When thetube diameter is large, a quantity of light which is radiated from onetube is large and hence, a quantity of light radiated directly above thetube is increased. However, in the dot pattern which has a limit indensity, the dot pattern cannot sufficiently block light from the lightsource having the large tube diameter and hence, a portion above thetube becomes light thus giving rise to brightness irregularities.Accordingly, it is difficult to achieve the uniformity of brightnesswith the high brightness light source unless not only the light curtainbut the arrangement position of the fluorescent lamp are taken intoconsideration.

Accordingly, it is an object of the present invention to provide atechnique which can acquire both of high efficiency and thin and uniformthickness in a liquid crystal display device which includes a directbacklight which is formed of a high brightness light source such as anHCFL.

The above-mentioned and other objects and novel features of the presentinvention will become apparent from the description of thisspecification and attached drawings.

To briefly explain the summary of typical inventions among theinventions disclosed in this specification, they are as follows.

In a liquid crystal display device which includes a liquid crystal paneland a backlight unit which is arranged on a side of the liquid crystalpanel opposite to a display screen of the liquid crystal panel, thebacklight unit includes a housing, a plurality of light sources arrangedin the inside of the housing, and a diffusion plate which is arrangedbetween the plurality of light sources and the liquid crystal panel, thediffusion plate includes a plurality of light blocking regions atpositions corresponding to the plurality of respective light sources,and the light blocking region at a center portion of the housing and thelight blocking region at an edge portion of the housing exhibitstransmissivities different from each other.

Further, in a liquid crystal display device which includes a liquidcrystal panel and a backlight unit which is arranged on a side of theliquid crystal panel opposite to a display screen of the liquid crystalpanel, the backlight unit includes a housing, a plurality of lightsources arranged in the inside of the housing, a diffusion plate whichis arranged between the plurality of light sources and the liquidcrystal panel, and an intermediate plate which is formed between theplurality of light sources and the diffusion plate, the intermediateplate includes a plurality of light blocking regions at positionscorresponding to the plurality of respective light sources, and thelight blocking region at a center portion of the housing and the lightblocking region at an edge portion of the housing exhibitstransmissivities different from each other.

Further, the light source is a hot cathode fluorescent lamp (HCFL).

The light blocking regions are formed by crest-like prisms which areparallel to the longitudinal direction of the light sources.

Further, the plurality of light blocking regions are formed of aplurality of rectangular reflection patterns and the reflection patternforming area is set narrower in the light blocking region at the housingedge portion than the light blocking region at the center portion of thehousing.

According to the present invention, even when the light source of highbrightness such as HCFL is used, it is possible to provide a liquidcrystal display module having high brightness uniformity.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a perspective view of a liquid crystal display module of thepresent invention;

FIG. 2 is a cross-sectional view of FIG. 1;

FIG. 3 is a view showing the distribution of light of respectiveconstitutional elements of this embodiment;

FIG. 4 is a top plan view of a diffusion plate 3;

FIG. 5 is a view showing details of a light blocking region 7 (abovefluorescent lamp on the center side);

FIG. 6 is a view showing details of a light blocking region 7 (abovefluorescent lamp on an edge side);

FIG. 7 is a view showing details of a light blocking region 7 (abovefluorescent lamp on the center side);

FIG. 8 is a view showing details of a light blocking region 7 (abovefluorescent lamp on an edge side);

FIG. 9 is a view showing another example of the light blocking region 7;

FIG. 10 is a view showing details of a light blocking region 7 (abovefluorescent lamp on the center side);

FIG. 11 is a view showing details of a light blocking region 7 (abovefluorescent lamp on an edge side); and

FIG. 12 is a cross-sectional view of the embodiment which includes anintermediate plate 9.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, embodiments of the present invention are explained indetail in conjunction with drawings.

Here, in all drawings for explaining the embodiments, parts havingidentical functions are given same symbols and their repeatedexplanation is omitted.

FIG. 1 is a perspective view showing the main constitution of a liquidcrystal display module of the embodiment of the present invention, andFIG. 2 is a cross-sectional view of FIG. 1.

In these drawings, numeral 1 indicates a liquid crystal panel, numeral 2indicates an optical film such as a prism sheet or a diffusion sheet,numeral 3 indicates a diffusion plate, numeral 4 indicates a housing inwhich a light source 5 is mounted. A plurality of light sources 5 aremounted in the housing 4, in this embodiment, an HCFL is used as thelight source 5. In case of a display of 32 inches, 4 to 6 pieces oftubes having diameter of 16 mm are mounted in the housing 4. In thisembodiment, an example in which 5 pieces of tubes are mounted in thehousing 4 is described. Further, in the inside of the housing 4, areflection sheet 6 which reflects light from the light sources 5 isarranged. Light blocking regions 7 are formed on the diffusion plate 3corresponding to mounting positions of the above-mentioned light sources5.

Here, the distribution of light of the respective constitutionalelements of this embodiment is shown in FIG. 3. This drawing shows onlya half from the center to an end portion. The constitution of anopposite side not shown in the drawing also has the same constitution insymmetry.

FIG. 3(D) shows the light sources, wherein distances between respectivefluorescent lamps and a distance between the fluorescent lamp and anedge of the housing are indicated as L1 to L3. In this embodiment, thesedistances are set to a substantially equal pitch. To be more specific,L1=L2≅L3 (L3>L2/2). It is needless to say that there may be a case thatthe distances is set to satisfy a relationship of L1≠L2≠L3, andparticularly, L1<L2, L1≠L2≠L3. For example, when the HCFL is used as thefluorescent lamp, the distances are set such that L1=65 mm, L2=67 mm,and L3=68 mm. To the contrary, when a CCFL is used as the fluorescentlamp, the distances are set such that L1=L2≅L3=approximately 20 to 25mm. The illumination distribution when the light radiated from theselight sources arrives at the diffusion plate 3 is shown in FIG. 3(C).Luminances above the center fluorescent lamp and above the fluorescentlamps adjacent to the center fluorescent lamp exhibit a shape whichsubstantially conforms to a cross section of the tube. However, withrespect to the illumination above the fluorescent lamp on the edge side,there is no fluorescent lamp close to the edge side and a space spreadsand hence, the luminance is gradually lowered toward the edge side. Thediffusion plate 3 is provided for overcoming such irregularities ofluminance. FIG. 3(B) shows the transmissivity distribution of thediffusion plate 3. Further, FIG. 3(A) shows the brightness distributionof the light which arrives at the liquid crystal panel 1. Thetransmissivity distribution of the diffusion plate 3 is configured topossess the brightness distribution such that the brightness in thevicinity of the center is high as shown in FIG. 3(A).

To be more specific, the transmissivity (FIG. 3(B)) of the diffusionplate 3 of this embodiment is configured to possess not only thedistribution which reverses the contrast of the illuminance distributionshown in FIG. 3(C) but also possesses transmissivities which differbetween the position above the center fluorescent lamp and the positionabove the fluorescent lamp on the edge side. Accordingly, widths andtransmissivities of the plurality of light blocking regions 7 formed onthe diffusion plate 3 are formed with adjustment at respective regionsto realize the transmissivities shown in FIG. 3(B).

The HCFL used in this embodiment exhibits the larger distance than theCCFL. Further, the HCFL radiates more light from one fluorescent lampthan the CCFL. Accordingly, the brightness is extremely increased rightabove the tube. Further, as described in this embodiment, from anoptical point of view, the tube is arranged close to the center(according to the rough approximation, an optical system is folded bythe reflection sheet 6 and hence, the tubes are optically uniformlyarranged such that L1=L2, L3=L1/2). Accordingly, there exists a drawbackthat a periphery of the edge portion becomes dark. A technique whichovercomes this drawback is explained hereinafter.

FIG. 4 is a top plan view of the diffusion plate 3. As shown in thedrawing, the light blocking region 7 is formed right above thefluorescent lamps corresponding to the number of the fluorescent lamps.The respective light blocking regions 7 can be realized by forming thediffusion plate 3 such that the transmissivities shown in FIG. 3(B) areacquired. For example, the light blocking region 7 at the center isformed to acquire the transmissivity of a portion (a) in FIG. 3(B), andthe light blocking region 7 on the edge side is formed to acquire thetransmissivity of a portion (b) in FIG. 3(B). Further, althoughdistances are formed between the respective light blocking regions 7 inthis drawing, the distances are not always necessary and the respectivelight blocking regions 7 may be continuously formed.

Hereinafter, examples of light blocking regions 7 for realizing thetransmissivities shown in FIG. 3(B) are explained as embodiments 1 to 4.

Embodiment 1

As an embodiment 1, the detail of the light blocking regions 7 used inthis embodiment is explained in conjunction with FIG. 5 and FIG. 6. FIG.5 is an enlarged view of the light blocking region 7 at a position SA1(above a center-side fluorescent lamp) shown in FIG. 4, and FIG. 6 is anenlarged view of the light blocking region 7 at a position SA2 (above anedge-side fluorescent lamp) shown in FIG. 4. FIG. 5(A) and FIG. 6(A) aretop plan views, and FIG. 5(B) and FIG. 6(B) are cross-sectional views.In the light blocking region 7 of this embodiment, the diffusion plate 3is formed into a prism shape in which a plurality of crest shapes arecontinuously connected, wherein a size (width) of the crest differsbetween a center portion and a peripheral portion of the diffusion plate3. Further, lateral straight lines (ridges) of respective crests are setto an equal length.

In FIG. 5, a large number of crests having the small width are formed,while in FIG. 6, a large number of crests having the wide width areformed. Although the light blocking region 7 above the fluorescent lampbetween the position SA1 (above the center-side fluorescent lamp) andthe position SA2 (above the edge-side fluorescent lamp) is not shown, byforming the light blocking region into a prism having a crest shape of asize equal to the size of the prism at the position SA1 or a sizebetween the sizes of the prisms at the positions SA1, SA2, it ispossible to ensure the continuity of the transmissivity.

Due to such a constitution, it is possible to change the transmissivitybetween the position SA1 above the center-side fluorescent lamp and theposition SA2 above the edge-side fluorescent lamp.

As can be also understood from FIG. 6, the shape of the light blockingregion 7 on the position SA2 side is formed in left-and-right asymmetrywith respect to the light source 5. Particularly, the light blockingregion 7 is formed in the crest shape which widely extends to the edgeside. Due to such a constitution, it is possible to realize the highertransmissivity at the edge side.

Further, with respect to the shapes of the respective crests, the ridgesmay be formed not only in a straight line but also in a line whichchanges a curvature thereof. For example, the ridges may be formed intoa spherical lens shape or an aspherical lens shape.

Embodiment 2

Next, as an embodiment 2, another example of the light blocking region 7is shown. FIG. 7 is an enlarged view of the light blocking region 7 atthe position SA1 (above a center-side fluorescent lamp) shown in FIG. 4,and FIG. 8 is an enlarged view of the light blocking region 7 at theposition SA2 (above an edge-side fluorescent lamp) shown in FIG. 4. Inthis embodiment, the light blocking region 7 is formed such that lengthsof left and right ridges of each crest differ from each other. That is,the ridge on the center side is long and the ridge on the peripheralside is short.

Further, in this embodiment, in the same manner as the embodiment 1, alarge number of crests having the small width are formed in FIG. 7,while a large number of crests having the wide width are formed in FIG.8. Due to such a constitution, it is possible to change thetransmissivity between the position SA1 above the center-sidefluorescent lamp and the position SA2 above the edge-side fluorescentlamp.

As can be also understood from FIG. 8, the shape of the light blockingregion 7 on the position SA2 side is, in the same manner as theembodiment 1, formed in left-and-right asymmetry with respect to thelight source 5, wherein the light blocking region 7 is formed in thecrest shape which widely extends to the edge side. Due to such aconstitution, it is possible to realize the higher transmissivity at theedge side.

Embodiment 3

Next, as the embodiment 3, another example of the light blocking region7 is shown in FIG. 9. In the above-mentioned embodiments 1, 2, the lightblocking region 7 is formed of the prism having the crest shape only incross section in the direction perpendicular to the fluorescent lamp. Inthis embodiment, the light blocking region 7 is formed of a prism havinga crest shape in cross sections in two directions. That is, in theperpendicular direction as well as in the parallel direction withrespect to the fluorescent lamp.

Also in this embodiment, by forming a large number of crests having asmall width at the position SA1 (above the center-side fluorescent lamp)and a large number of crests having a wide width at the position SA2(above the edge-side fluorescent lamp), it is possible to change thetransmissivity between the position SA1 above the center-sidefluorescent lamp and the position SA2 above the edge-side fluorescentlamp.

Further, the prism shape can be formed two-dimensionally and hence, thenumber of faces which reflect light is large whereby the furtheruniformity can be expected.

Embodiment 4

Next, as the embodiment 4, another example of the light blocking region7 is explained in conjunction with FIG. 10 and FIG. 11. In theabove-mentioned embodiments 1 to 3, the example which forms the lightblocking region 7 into the prism shape is explained. In this embodiment,the position distribution of transmissivity is controlled based on areagray scales of a reflection pattern 8 by forming the reflection pattern8 made of aluminum or the like on the diffusion plate 3 by vapordeposition. Here, provided that a material of the reflection pattern 8exhibits high reflectance, any material can be used. FIG. 10 is a viewshowing the light blocking region 7 at the position SA1 (above acenter-side fluorescent lamp), and FIG. 11 is a view showing the lightblocking region 7 at the position SA2 (above an edge-side fluorescentlamp). By forming a large number of reflection patterns 8 having a widewidth at the position SA1 (above the center-side fluorescent lamp) and alarge number of the reflection pattern 8 having a narrow width at aposition SA2 (above the edge-side fluorescent lamp), it is possible tochange the transmissivity between the position SA1 above the center-sidefluorescent lamp and the position SA2 above the edge-side fluorescentlamp.

As can be also understood from FIG. 11, the shape of the light blockingregion 7 on the position SA2 side is formed in left-and-right asymmetrywith respect to the light source 5. Particularly, the narrowerreflection pattern 8 is formed at the edge side. Due to such aconstitution, it is possible to realize the higher transmissivity at theedge side. Further, to enhance the transmissivity at the edge side, thereflection pattern 8 on a side closer to the edge than the light source5 in FIG. 11 may not be formed.

In the embodiments explained heretofore, the examples which form thelight blocking region 7 on the diffusion plate 3 arranged right belowthe optical sheet 2 are shown. Next, a constitutional example other thanthe above-mentioned example is explained as an embodiment 5.

Embodiment 5

The constitution of this embodiment is shown in FIG. 12. In thisembodiment, an intermediate plate 9 is newly arranged between adiffusion plate 3 and a light source 5, and light blocking regions 7 areformed on the intermediate plate 9. The intermediate plate 9 per se isformed of a material having a high transparency (an acrylic plate, adiffusion plate having a high total light transmissivity or the like)and, at the same time, the intermediate plate 9 is arranged in aspaced-apart manner from the diffusion plate 3 and the light source 5.The relationship between a total light transmissivity T1 of thediffusion plate 3 and a total light transmissivity T2 of theintermediate plate 9 except for the light blocking regions 7 is set toT2>T1. To be more specific, the total light transmissivity T1 is 50 to60%, and the total light transmissivity T2 is approximately 70%. Thisrelationship is adopted for allowing light which is reflected on areflection sheet 6 arranged in the inside of a housing 4 to be radiatedto an intermediate space defined between the light sources from theintermediate plate 9 as much as possible. The constitutions shown in theabove-mentioned embodiments 1 to 4 may be applicable to a shape of thelight blocking regions 7. Further, the closer the intermediate plate 9is arranged to the light source 5, the light blocking region 7 can bearranged closer to the light source 5 and hence, the brightness can bemade uniform to some extent at a position close to the light source 5and, at the same time, the brightness can be made further uniformbetween the intermediate plate 9 and the diffusion plate 3. For example,when the HCFL having a diameter of 16 mm is used as the light sources 5,by setting a distance between the fluorescent lamp and the intermediateplate 9 to approximately 3 mm, the transmissivity distribution of therespective light blocking regions 7 may be set uniform. In thisembodiment, however, to spread the light which is already made uniformto some extent at the intermediate plate 9 between the intermediateplate 9 and the diffusion plate 3 (to enable the radiation of light to aremote place), it is necessary to ensure some distance. To be morespecific, when the HCFL having a diameter of 16 mm is used as the lightsources 5, the distance of 10 mm or more becomes necessary.

In this embodiment, the light blocking regions 7 are arranged close tothe light sources 5 and hence, a range that a viewer can directlyobserve the light sources 5 when the viewer observes in the obliquedirection becomes narrow. Accordingly, this embodiment is advantageousfor maintaining the brightness uniformity in any viewing angle.

Further, in this embodiment, in addition to the insertion of diffusionplate 3 between the light sources 5 and the liquid crystal panel, theintermediate plate 9 is inserted between the light sources 5 and theliquid crystal panel and, at the same time, the distance is ensuredbetween the intermediate plate 9 and the diffusion plate 3 and hence,the light can be made uniform in two stages. Accordingly, a uniformlight acquisition effect of this embodiment is large and hence, thereduction of thickness of the liquid crystal display module can berealized. To be more specific, in the embodiment which is explained inconjunction with FIG. 2, it is necessary to set a distance from thebottom surface of the housing 4 to the diffusion plate 3 to 40 mm.However, in this embodiment, the distance from the bottom surface of thehousing 4 to the diffusion plate 3 can be set to 30 mm.

Although the invention made by inventors of the present invention hasbeen specifically explained in conjunction with the embodimentsheretofore, it is needless to say that the present invention is notlimited to the above-mentioned embodiments and various modifications areconceivable without departing from the gist of the present invention.

1. A liquid crystal display device comprising: a liquid crystal panel;and a backlight unit which is arranged on a side of the liquid crystalpanel opposite to a display screen of the liquid crystal panel, whereinthe backlight unit includes: a housing, a plurality of light sourcesarranged in the inside of the housing; a diffusion plate which isarranged between the plurality of light sources and the liquid crystalpanel; and an intermediate plate which is formed between the pluralityof light sources and the diffusion plate, and the intermediate plateincludes a plurality of light blocking regions at positionscorresponding to the plurality of respective light sources.
 2. A liquidcrystal display device according to claim 1, wherein the light source isa hot cathode fluorescent lamp (HCFL).
 3. A liquid crystal displaydevice according to claim 1, wherein respective widths of the pluralityof light blocking regions are set larger than a radial width of thelight sources.
 4. A liquid crystal display device according to claim 1,wherein the transmissivity differs between the light blocking region ata center portion of the housing and the light blocking region at an edgeportion of the housing.
 5. A liquid crystal display device according toclaim 1, wherein crest-like irregularities arranged parallel to thelongitudinal direction of the light source are formed on the lightblocking region.
 6. A liquid crystal display device according to claim5, wherein the crest shape of the light blocking region is a crest shapewhich has a narrow width at a center portion and a wide width at aperipheral portion.
 7. A liquid crystal display device according toclaim 5, wherein respective crests of the irregularities of the lightblocking region have a long ridge at a center side and a short ridge ata peripheral side.
 8. A liquid crystal display device according to claim5, wherein a crest shape of the light blocking region at the edgeportion of the housing is formed such that a housing-center-side shapeand a housing-edge-side shape are formed in asymmetry providing thelight source as a center, and the housing-edge-side exhibits the highertransmissivity.
 9. A liquid crystal display device according to claim 1,wherein the plurality of light blocking regions form a plurality ofrectangular reflection patterns, and an area for forming the reflectionpatterns is narrower in the light blocking region at the edge portion ofthe housing than in the light blocking region at the center portion ofthe housing.
 10. A liquid crystal display device according to claim 1,wherein the light blocking region at the center portion of the housingand the light blocking region at the edge portion of the housing havethe same transmissivity.
 11. A liquid crystal display device accordingto claim 1, wherein a total light transmissivity of the intermediateplate except for the light blocking region is higher than a total lighttransmissivity of the diffusion plate.